This invention relates to non-metallurgical attachment of dissimilar metals, and in particular, but not by way of limitation, to a method and an apparatus for non-metallurgically attaching corrosion resistant metals to non-corrosion resistant metals.
Heavy industries, such as utility power generation companies, other solid fossil fuel burning plants, chemical processing plants, etc., release corrosive gases through FGD (flue-gas desulfurization) systems composed of ducts and ductworks such as scrubbers and precipitators prior to ultimate release through stacks. Such ducts, duct-works and stacks are commonly made of non-corrosion resistant metals such as carbon steel. Through heavy use by passage of flue-gases, the metals that comprise these substrate ducts and stacks are constantly corroded through chemical reactions and eroded through physical passage of flue-gases, resulting in fissures, cracks, and dangerous cavities. Thus, some form of protection is required if such ducts and stacks are to last a cost-effective period.
Furthermore, the heavy industries mentioned above and others have recently been restricted in their release of nitrous oxides (NO.sub.x) and sulfuric acids (SO.sub.2) by regulatory agencies. For instance, the E.P.A. has issued stringent restrictions on the amounts of NO.sub.x and SO.sub.2 that any given plant can release into the atmosphere. These regulations require these industries to modify existing pollution control techniques to comply with current federal standards. Conduit and stack systems have a design-to-construction lag time of about ten years. Because it takes approximately ten years from the conception of a system to actual commercial on-line application, systems designed before the onset of recent emission restrictions fail to comply with the new regulations.
As a result, older plants have had to use additives to reduce the release of NO.sub.x and SO.sub.2 because ad hoc structural design changes in existing systems is impractical if not impossible. However, the earlier design systems of these older plants did not necessarily factor into consideration the use of additives which reduce emission of NO.sub.x and SO.sub.2. The current use of these additives by older plants can compound corrosion problems, making the need for an improved corrosion resistant lining even more necessary.
In the past, various linings have been used to retard this constant wear and tear of substrate metal by corrosion with refractory lining materials or paint-like coatings. One such refractory lining used is Gunite which helps resist corrosion when applied in approximately two-inch thick coats to non-corrosion resistant metals such as carbon steel. However, such linings prove uneconomical because they do not last and have to be constantly replaced. Not only are these materials expensive, costly manpower is needed to replace and maintain the linings. Furthermore, these coatings are inadequate in ensuring protection because the coatings themselves are not immune to corrosion. They merely act as sacrificial linings to slow the degradation process down.
Corrosion resistant metal linings are a practical improvement over these application difficulties. Metal alloys such as hastelloys (high grade nickel alloys) and metals such as titanium are resistant to both corrosion and erosion. They can be permanently clad onto the inner surfaces of ductwork and stacks and can last throughout the lifetime of the non-corrosion resistant metal substrate that comprise these conduits.
Hastelloys are usually cost-prohibitive because of wide fluctuations in their price which is dependant on swings in the geopolitical climate. Nickel is produced in countries whose political policies are not notably stable and long-term cost planning and projections of supply are impractical. Titanium is the corrosion resistant metal of choice because of the stability of its geopolitical availability and the relative stability of its price. Furthermore, titanium is one-half the weight of hastelloys, and since the application technology involves manual installation of the sheets of metal, the lighter weight provides a relatively easier installation.
However, regardless of availability, costs, and effective corrosion resistance of either alloys or pure metal, the real problem encountered by the relevant industry is the difficulty in the method of affixing corrosion resistant metals, such as titanium, to corrosion susceptible substrate surfaces to form a complete protective seal.
Corrosion resistant metals have to completely cover the surface area of non-corrosion resistant metals; otherwise, destruction will persist through cracks and minute openings in the FGD systems where corrosive materials and moisture condensate can creep into. Ideally, corrosion resistant metal sheets need to be welded directly onto the non-corrosion resistant metal substrate forming tight bonds so that a complete, leakproof seal is formed. However, metallurgically, metals of dissimilar types such as titanium and carbon steel cannot be welded together.
There is a need for a method of attaching such dissimilar metals without metallurgical attachment, while still maintaining the ability to cover and completely seal off the total surface area of metal substrates susceptible to corrosion.
An industrial group called Pfaudler has disclosed a method of attaching titanium to carbon steel in a publication called Resista-Clad Physical & Chemical Performance Data, Pfaudler Data Sheet DS49-303-1, copyright Sohio Chemicals 1985. Basically, the method involves a brazing technique utilizing an electrode wheel. A carbon steel edge is fused to a sheet of titanium using silver solder by applying electrode heat and physical pressure using the electrode wheel. The Data Sheet claims the process to be "resistance welding"; however, it is well-known to those skilled in the art that it is impossible to metallurgically attach carbon steel to titanium.
Application by the relevant industry using the Pfaudler method has seen failures such as delamination due to the brittleness of the resulting brazing attachment. The brazing technique is inconsistent in its binding strength. When applied within ductwork, the attachments between carbon steel and titanium using the Pfaudler method form hairline cracks or simply delaminate and fall apart. Thus, this process is practically no improvement over prior methods of simple coating with refractory materials.
Thus, there is a need in the relevant industry for an improved method and apparatus which can effectively clad a corrosion resistant metal such as titanium to a dissimilar metal substrate with sufficient strength and sealing ability to allow protection against erosion and corrosion. There is a need for a more cost-effective lining to increase the productivity of FGD systems to protect against the damages caused by flue-gases, which is compounded by additives that have to be used by plants whose original designs fail E.P.A. requirements for reduction of NO.sub.x and SO.sub.2 emissions.
The present invention is an improved and cost-effective method of permanently cladding corrosion resistant metals such as titanium to non-resistant metal substrates such as carbon steel without metallurgical welding of the two dissimilar metals using an apparatus called PERMA CLAD. Thus, the PERMA CLAD system of the present invention is an improved method and apparatus that addresses the above-mentioned needs. These improvements and other novel advances will become apparent to those skilled in the art by the following disclosure of the invention.